A system of this type, conforming to C.C.I.T.T. Recommendation V36 and utilizing the so-called Class IV partial-response technique, has been described for example in an article by Becker, Kretzmer and Sheehan entitled "A New Signal Format for Efficient Data Transmission", Bell System Technical Journal, May/June 1966. The carrier may have a frequency of, say, 100 KHz; the frequency spectrum of the sideband is of sinusoidal configuration and occupies a band of a width equal to half the bit cadence alongside the carrier.
At the receiver, the incoming message wave is subjected to amplitude demodulation under the control of the carrier which is separated from the remainder of that wave by filtering. The baseband signal recovered by the amplitude demodulator is then converted to binary form, corresponding to the original data pulses, with the aid of a local oscillation constituting a clock signal derived by a synchronization extractor, in the absence of a pilot tone, from the baseband signal itself. The latter signal, however, is subjected along the transmission path to distortion different from that undergone by the carrier, this resulting in spurious phase shifts therebetween. Such a phase error tends to narrow the eye diagram obtained when the baseband signal is visualized on the screen of an oscilloscope, thereby increasing the likelihood of a reading error due to phase jitter when that signal is sampled in the decoder at instants determined by the clock signal and timed to coincide with the center of the eye diagram.